A great deal of work has been directed to the drying of lumber products. Usually, the lumber products are stacked while providing the air passage by locating spacers between adjacent lumber products. Almost all of the drying methods involve at least two drying steps. Use is commonly made of large-sized buildings provided with heaters and fans. In fact such dryers did not provide good results due to the non-uniform circulation of the heated air through the stacks of lumber products. Further methods require a plurality of handling steps of selection of the lumber products which leads to expensive costs when operated.